In view of the high growth of wireless communication systems such as cellular mobile phones, portable wireless phones and global satellite position systems, the development of small and light portable mobile phones is a natural trend. Therefore, small microwave elements are highly desired in the industry.
Therefore, various multi-layered electronic devices have been developed to increase the volume efficiency to meet the above requirements. Also, it is desired that high frequency ceramic filters are multilayered and small in size. For the preparation of multilayered ceramic filters, the devices have to be cofired with low loss conductors such as silver and copper. Therefore, the development of materials which can be sintered at low temperatures into microwave ceramic devices with a high dielectric constant is highly desired.
In view of the trend for the miniaturization and modulization of high frequency microwave communication devices, it is very important to obtain materials which can be sintered at low temperatures into ceramics with a high dielectric constant. A microwave ceramic material has to have a high dielectric constant in order to meet the requirements for miniaturization, a high quality factor (i.e. low dielectric loss) to provide high-quality high-frequency signals.
The frequency of microwaves ranges from 0.9 to 300 GHz. The properties of selected microwave ceramic materials must meet the following requirements for miniaturization:
K: 10-90; PA1 Tan .delta.: &lt;10.sup.-3 ; PA1 TC.sub.f : .sup.- 10-10 MK.sup.-1 PA1 Sintering Temperature: 1360.degree. C.; PA1 Dielectric Constant (@ 7 GHz): 38; PA1 Quality Factor (@ 7 GHz): 7000; PA1 Temperature Coefficient: 0 ppm/.degree.C.
wherein K is the dielectric constant; Tan .delta. is the dielectric loss and TC.sub.f is the temperature coefficient. The size of ceramic filters decreases with increasing its dielectric constant. Furthermore, the less the dielectric loss, the better the quality factor and thus, the better the high-frequency signals. Moreover, the temperature coefficient should be close to zero so as to ensure that the change of resonance frequency with temperature is negligible.
There are various microwave ceramic materials such as CaZr.sub.0.985 Ti.sub.0.015 O.sub.3, BaTi.sub.4 O.sub.9, ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 and (Ba, Pb)Nd.sub.2 Ti.sub.5 O.sub.4. However, all these materials have sintering temperatures above 1300.degree. C. which is too high to cofire with highly conductive metals such as Ag and Cu. Approaches taken to reducing the sintering temperature to 1000.degree. C. or less include using fine powders prepared by chemical processes, adding glass into microwave ceramic materials, or using sintering flux.
U.S. Pat. No. 5,449,652 describes a microwave dielectric ceramic composition comprising Bi.sub.(2-x) (Zn.sub.(2+y)/3 Nb.sub.(4/3))O.sub.(7-3x/2+y/3) wherein 0.24&lt;x&lt;0.333, 0.120&lt;y&lt;0.3; and Bi.sub.(1-z) Ca.sub.(z) (Zn.sub.(2+y)/3 Nb.sub.(4/3))O.sub.(7-3x/2+y/3+xz/-z) wherein 0&lt;x&lt;0.667, 0&lt;y&lt;0.30, 0&lt;z&lt;0.2. The properties of the microwave dielectric ceramic composition are as follows: dielectric constant &gt;100; quality factor (@ 7 GHz)=7000; temperature coefficient=10 ppm/.degree.C.
U.S. Pat. No. 4,672,152 describes a ceramic composition with a low sintering temperature and a low dielectric constant which comprises 50-95 wt % crystallizable glass and 5-50 wt % ceramic filler. The dielectric system has a dielectric constant of 5.1-6.0. The crystallizable glass consists of 5-20 wt % lithium oxide, 60-90 wt % silicon dioxide, 1-10 wt % aluminum oxide, and 1-5 wt % alkaline metal oxide other than lithium oxide. The ceramic filler includes silicon dioxide and aluminum oxide.
U.S. Pat. No. 4,755,490 describes a ceramic composition with a low sintering temperature and a low dielectric constant which comprises 10-50 wt % alumina, 0-30 wt % fused silica, and 50-60 wt % of a frit comprised of 4 wt % CaO, 12 wt % MgO, 29 wt % B.sub.2 O.sub.3 and 42 wt % SiO.sub.2. The dielectric composition has a sintering temperature below 1000.degree. C., a dielectric constant of 4.5-6.1, and a linear thermal expansion coefficient of 3.9-4.2.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 5,415,945 describes a ceramic composition with a low sintering temperature and a high dielectric constant which comprises 75-85 mol % Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 +0-15 mol % PbTiO.sub.3 +5-16.5 mol % Pb(Zn.sub.1/2 W.sub.1/2)O.sub.3 +Pb(Cu.sub.1/3 Nb.sub.2/3)O.sub.3. The composition has a sintering temperature of 1000.degree. C. and a dielectric constant of 1000-4000.
U.S. Pat. No. 5,262,368 describes a ceramic composition with a low sintering temperature and a high dielectric constant which comprises BaTiO.sub.3, BaCuO.sub.2, WO.sub.3 and MoO.sub.3. The system has a sintering temperature of 1150.degree. C., a dielectric constant of 2000-3000 (1 KHz) and a dielectric loss of 2.5%-16% (1 KHz).
U.S. Pat. No. 5,461,014 describes a ceramic composition with a low sintering temperature and a high dielectric constant which comprises Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and BaCuO.sub.2. The system has a sintering temperature of 1050.degree. C., a dielectric constant of 7000-8000 (1 KHz) and a dielectric loss less than 3%.
ROC Patent Publication No. 159830 describes a ceramic composition with a high dielectric ceramic composition which is comprises Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. The system has a sintering temperature of less than 1000.degree. C. and a dielectric constant of 8000-10000.
In view of the above, it is obvious that ceramic materials which have a low sintering temperature, a high dielectric constant and a high quality factor (i.e. low loss) are needed in the industry. The present invention provides a way to meet this requirement.